Why does using my slave flash darken the image?

Why does my slave flash at full power actually seem to darken the image?

The camera (Lumix G1) is set to M-Mode, forced flash at -2, f6, 1/30s. I set my external flash (Nissin Di466 FT) to slave mode "S2" to ignore the first "measure" flash.

I do an experiment with three shots:

1) Slave flash off: Image is medium bright.

2) Slave flash +0.5: Image is brighter

3) Slave flash +1.5: Image is really dark with a red tone

How can this be? Even if the slave flash has missed the right point in time, image 3 should still have the same brightness like image 1, since the camera flash is still on. Am I missing something?

(iExposure is off, and ISO is set to 200).

[Edit: it turned out that on the Di466 "S2" is the mode that does NOT ignore the pre-flash. But I still do not understand the behavior.]

Answer

You guys are on the right track. Here's what's happening in each picture:

Slave flash off.

• Your camera fires a pre-flash


• The camera determines how much flash power it needs for proper exposure based on how bright the return flash is.


• The shutter opens, and the camera fires its flash at the power level it just determined.

You get a medium-bright picture.

Slave flash at +0.5

• Your camera fires a pre-flash, which triggers the slave flash.



• The camera sees a very bright return flash, so it decides to use very low power on-camera flash for the exposure.


• The shutter opens, and the camera fires its flash at a very low power, which triggers the slave flash again. At only +0.5 power, the slave flash still has enough charge to fire again immediately.

Although the on-camera flash was very dim, the slave is much brighter than is needed for proper exposure, so the picture is overexposed.

Slave flash at +1.5:

• Your camera fires a pre-flash, which triggers the slave flash.


• The camera sees a very bright return flash, so it decides to use very low power on-camera flash for the exposure.


• The shutter opens, and the camera fires its flash at a very low power. However, the slave flash, at +1.5 power, has already used more than half of the charge in its capacitors, so it can't fire again immediately.

Since the on-camera flash is dim, and the slave doesn't fire while the shutter is open, this picture is darker, and only consists of ambient light. Since your camera thought the scene was going to be lit mostly by flash, it balanced for xenon light. The ambient light (which is probably tungsten or tungsten-colored CFLs) is a much lower color temperature than xenon, so your picture has a red tint.